1. Field of the Invention
The present invention relates to a liquid crystal display device and also to a method for driving such a liquid crystal display device. More particularly, the present invention relates to a matrix type liquid crystal display device in which each picture element of the liquid crystal cell or panel is provided with a field effect transistor (hereinafter referred to as a FET), and also to a method for driving such a matrix type liquid crystal, thereby enabling the drive by multi-line multiplex operation and providing a high contrast picture.
2. Description of the Prior Art
A matrix type liquid crystal display panel employing FETs is developed by Westinghouse Electric Co. in 1973 wherein the matrix type liquid crystal display panel has picture elements each formed by FET and thin film capacitor.
A detail of Westinghouse's matrix type liquid crystal display panel and its driving method are disclosed in IEEE Transactions on Electron Devices, Vol. ED-20, No. Nov. 11, 1973, entitled, "A 6.times.6 Inch 20 Lines-per-Inch Liquid-Crystal Display Panel" by T. P. Brody et al.
According to Westinghouse's matrix type liquid crystal display panel, there are following problems:
(1) In a case where a drive signal that illuminates all the picture elements other than one element selected by i(th) source electrode Si and j(th) gate electrode Gj, wherein i and j are integers, is applied, a drain of a FET provided in that one element receives an effective voltage which is equal to or above a voltage needed to turn on the FET, resulting in error display or in different contrast that varies with respect to the change of number of illuminating elements.
(2) Since the voltage-current characteristic of the FET is non-symmetric between positive and negative regions, the voltage applied to the liquid crystal has a waveform which is non-symmetric between positive and negative regions and, therefore, the liquid crystal receives a voltage having a d.c. component. This results in short life time of the liquid crystal.
In order to solve the above problems (1) and (2), there have been proposed an improved liquid crystal display panel and its driving method which are disclosed in U.S. Pat. No. 4,386,352, issued May 31, 1983 and also in U.S. Pat. No. 4,385,292, issued July 25, 1980 which are assigned to the same applicant as the present application.